1. Field of the Invention
This invention relates to a coating method and a coating apparatus, and particularly to a coating method capable of uniformly coating both sides of a web with a coating liquid, and a coating apparatus therefor.
2. Description of Related Art
Various kinds of coating methods such as a roll coat method, a gravure coat method, a slide bead coat method, a doctor coat method, etc. have been conventionally used as a method of coating liquid on a web. Of these methods, an extrusion coating method has recently attracted public attention thereto because it has high productivity, excellent operating performance and has excellent controllability of the thickness of coated films.
In the extrusion coating method, a coating liquid is coated on a web using a coating head while a front edge surface (at the upstream side of the web feeding direction) and a back edge surface (at the downstream side of the web feeding direction) of the coating head are pushed against the web between web feeding means such as a pair of support rolls or the like so that predetermined tension is applied to the web. In this case, the clearance between the back edge surface and the web is controlled to vary in accordance with variation of an extrusion amount of the coating liquid which is extruded through a slit of the coating head, thereby setting the thickness of the coating liquid on the web to a desired one.
In order to perform an uniform coating operation on a web using the extrusion coating method as described above, a coating apparatus in which the back edge surface (at the downstream side of the web feeding direction) of a coating head is designed to have a triangular shape in section was proposed, and this apparatus is disclosed in Japanese Laid-open Patent Application No. 57-84771. Further, Japanese Laid-open patent application No. 58-109162 discloses a coating apparatus in which proper restriction is imposed on an angle at which each of the front edge surface and the back edge of the coating head intersects to a line connecting the apexes of the edges of both faces at the slit side, and Japanese Laid-open Patent Application No. 60-238179 also discloses a coating apparatus in which the back edge surface (at the downstream side of the web feeding direction) of the coating head is curved and an intersecting angle between the back edge surface and the front edge surface or the like is restricted.
When the coating liquid is coated on both sides of a web using the extrusion-type coating head as described above, the double-coated web cannot be supported at both sides thereof in contact with a support roll or the like, and thus the web must be dried in a drying apparatus using a non-contact drying treatment (process) in which air is blown against both sides of the web to support the web in a non-contact state. However, in the non-contact drying process as described above, vibrations are induced in the web even when the balance of air blown against the web is minutely varied. The vibration thus induced affects the coating head which is located nearest to the non- contact drying apparatus, resulting in unevenness of the thickness of the coated film. Therefore, it is difficult to perform a uniform double-side coating operation to obtain uniform thickness on the whole surface of the web.
In order to solve the above problem there has been proposed a vibration absorbing device which has two air ejection chambers each containing a distributor and which serves to form thin air films on both sides of a web. In this case, the vibration absorbing device is disposed between the non-contact drying apparatus and the coating head located nearest the non-contact drying apparatus, and vibration induced in the web is absorbed by the non-contact drying operation, thereby preventing the coating operation of the coating head from being disturbed by the vibrations (see Japanese Laid-open Patent Application No. 62-244469). In addition, a solid smoother is further disposed between the vibration absorbing device as described above and the coating head so as to be in contact with the coated film of the web (see Japanese Laid-open Patent Application No. 62-247861).
However, the coating apparatus as disclosed in Japanese Laid-open Patent Application No. 57-84771 has the following problem. That is, the back edge surface (at the downstream side of the web feeding direction) has a triangular shape in section, and thus it has an apex (in section). Therefore, it is difficult for dust or foreign matter to flow out beyond the apex, and thus streaking on a coated film is liable to occur due to the dust or foreign matter which is trapped at the outlet of a slit which is provided in the coating head applying the coating liquid to the web.
Further, the coating apparatus as disclosed in the Japanese Laid-open Patent Application No. 58-109162 and the coating apparatus as disclosed in the Japanese Laid-open Patent Application No. 60-238179 have a problem that these coating apparatuses are liable to be adversely affected by vibrations occurring in the web through the non-contact drying process and thus it is difficult to form a uniform coated film. Therefore, it is required that the vibration absorbing device and the solid smoother as disclosed in the Japanese Laid-open Patent Application Nos. 62-244469 and 62-247861 are disposed between the non-contact drying apparatus and the coating head. However, in this case the manufacturing cost is increased because surplus equipment such as the vibration absorbing device and the solid smoother must be provided. In addition, when the solid smoother is contacted with the coated film on the first surface of the web for the double-side coating operation as disclosed in the Japanese Laid-open Patent Application No. 62-247861, the contact between the solid smoother and the coated film induces variation of the thickness of the coated film on the first surface of the web.
Still further, in the double-side coating technique as disclosed in the Japanese Laid-open Patent Application No. 58-109162. two coating heads are respectively disposed on the both sides of the web in an area where the tension of the web is controlled, and thus it is difficult to properly distribute desired web tension to each coating head. In addition, the angle adjustment between each coating head and the web cannot be independently and solely performed for each coating head, so that a coated film having optimum thickness cannot be uniformly formed on both sides of the web.